fix(threads): add hint comments

Author: manyinsects

exercises/threads/threads1.rs

@@ -1,5 +1,5 @@
 // threads1.rs
-// Make this compile and run! Execute 'rustlings hint threads1' for hints :)
+// Execute `rustlings hint threads1` or use the `hint` watch subcommand for a hint.
 // This program should wait until all the spawned threads have finished before exiting.
 
 // I AM NOT DONE

exercises/threads/threads2.rs

@@ -1,5 +1,5 @@
 // threads2.rs
-// Make this compile! Execute `rustlings hint threads2` for hints :)
+// Execute `rustlings hint threads2` or use the `hint` watch subcommand for a hint.
 // Building on the last exercise, we want all of the threads to complete their work but this time
 // the spawned threads need to be in charge of updating a shared value: JobStatus.jobs_completed